Systematic Tuning of Electronic Band Gap with Varying Dopant Concentration in Zinc Gallate Nanoparticles

Rare-earth (RE3+ = Eu3+, Tb3+, Sm3+, and Er3+) doped ZnGa2O4 nanoparticles were synthesized using an eco-friendly aloe-vera-assisted green combustion method. The rapid exothermic reaction produced fine nanocrystalline powders, which were subsequently calcined at 600°C for improved crystallinity. PXRD confirmed the formation of a pure cubic spinel phase with no impurity peaks, while slight peak shifts and broadening indicated the presence of lattice strain and nanoscale crystallite dimensions. The combustion-derived powders exhibited agglomerated, irregular morphologies typical of gas-evolving reactions. Optical diffuse reflectance spectroscopy revealed a clear dependence of the energy band gap (Eg) on both crystallite size and dopant concentration. For all RE dopants, the Eg decreased progressively with increasing particle size, reflecting the relaxation of quantum-confinement effects. Similarly, Eg showed a systematic decrease with higher dopant mol%, attributed to dopant-induced lattice distortion, creation of defect states, and modification of local electronic environments within the ZnGa2O4 matrix. Among the dopants, Er3+ exhibited the highest Eg in the undoped limit, while Tb3+ produced the strongest band-gap narrowing at higher concentrations. These results demonstrate that RE doping combined with size control offers an effective strategy for tailoring the optical band structure of ZnGa2O4, underscoring its potential for photonic, sensing, and optoelectronic applications.